The present invention relates to turbine buckets having airfoil tip shrouds and, particularly, to a shroud for a stage 2 bucket having a tip seal extending between opposite ends of the shroud, with a cutter tooth located along said tip seal, substantially centered between the opposite ends of the shroud in the direction of rotation of the bucket.
Airfoils on turbine buckets are frequently provided with tip shrouds. The shroud prevents failure of the airfoil in high cycle fatigue due to vibratory stresses. A tip shroud seal typically projects radially outwardly from the outermost surface of the shroud, and extends circumferentially between opposite ends of the shroud in the direction of rotation of the turbine rotor. The tip shroud seal conventionally extends radially into a groove formed in a stationary shroud opposing the rotating tip shroud. In some designs, the stationary shroud has a honeycomb pathway. Rather than providing a zero tolerance seal between the tip shroud and the stationary shroud, resulting in instability of the airfoil, it has been found desirable to provide a leakage path over the tip shroud seal which will remove such instability. Typically, a cutter tooth is provided at the leading edge of the tip shroud seal so as to cut a wider groove in the honeycomb pathway of the stationary shroud than the width of the tip shroud seal. This enables leakage flow between the high and low pressure regions on opposite sides of the tip shroud seal within  the groove. While this results in an undesirable decrease in pressure drop across the airfoil with resulting diminishment of sealing capability, the lost efficiency is compensated by an increase in the stability of the airfoil.
Because the mass of the tooth is not located in the same radial line as the center of mass of the airfoil, however, it has been discovered that this asymmetrical design increases the stresses in the fillet below the shroud on the bucket (i.e., in the region between the airfoil and the tip shroud) particularly at high temperatures. This increased stress at high temperatures leads to a high creep rate and ultimately can result in failure of the shroud, for example, by cracking or splitting. It will be appreciated that the failure of a single bucket shroud causes the turbine necessarily to be taken off-line. Consequently, shroud failure due to increased stress at the fillet region between the tip shroud and the airfoil requires time-consuming and costly repairs, including bringing the turbine off-line, in addition to the labor and replacement parts necessary to effect the repair.